无涯教程-Derby - Group By语句

无涯教程
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GROUP BY子句与SELECT语句一起使用，如果数据相同，则用于形成子集，通常，此子句后跟ORDER BY子句，并放在WHERE子句之后。

GROUP BY - 语法

以下是GROUP BY子句的语法-

ij>SELECT column1, column2, . . . table_name GROUP BY column1, column2, . . .;

GROUP BY - 命令行示例

假设无涯教程在数据库中有一个名为Employees的表，其中包含以下记录-

ID |NAME |SALARY |LOCATION
------------------------------------------------------------------
1 |Amit |30000 |Hyderabad
2 |Rahul |39000 |Lucknow
3 |Renuka |50000 |Hyderabad
4 |Learnfk |15000 |Vishakhapatnam
5 |Kalyan |40000 |Hyderabad
6 |Trupthi |45000 |Vishakhapatnam
7 |Raghav |12000 |Lucknow
8 |Suchatra |33000 |Vishakhapatnam
9 |Rizwan |20000 |Lucknow

以下带有GROUP BY子句的SELECT语句根据位置对表进行分组，它显示给某个位置的员工的工资总额。

ij> SELECT Location, SUM(Salary) from Employees GROUP BY Location;

这将生成以下输出-

LOCATION |2
-------------------------------------------------------
Hyderabad |120000
Lucknow |71000
Vishakhapatnam |93000
3 rows selected

以相同的方式，以下查询查找在某个位置作为员工薪水所花在员工身上的平均金额。

ij> SELECT Location, AVG(Salary) from Employees GROUP BY Location;

这将生成以下输出-

LOCATION |2
-----------------------------------------------------
Hyderabad |40000
Lucknow |23666
Vishakhapatnam |31000
3 rows selected

GROUP BY - JDBC示例

以下JDBC示例演示了如何使用 Group By 子句以及如何使用JDBC程序在Apache Derby中的表上执行CURD操作，在这里，无涯教程使用嵌入式驱动程序连接到名为sampleDB的数据库(如果不存在则创建)。

import java.sql.Connection;
import java.sql.DriverManager;
import java.sql.Statement;
import java.sql.ResultSet;
public class GroupByClauseExample {
   public static void main(String args[]) throws Exception {
      //注册驱动
      Class.forName("org.apache.derby.jdbc.EmbeddedDriver");

  </span><span class="com">//创建连接</span><span class="pln">
  </span><span class="typ">String</span><span class="pln"> URL</span><span class="pun">=</span><span class="str">"jdbc:derby:sampleDB;create=true"</span><span class="pun">;</span><span class="pln">
  </span><span class="typ">Connection</span><span class="pln"> conn</span><span class="pun">=</span><span class="typ">DriverManager</span><span class="pun">.</span><span class="pln">getConnection</span><span class="pun">(</span><span class="pln">URL</span><span class="pun">);</span><span class="pln">

  </span><span class="com">//获取Statement对象</span><span class="pln">
  </span><span class="typ">Statement</span><span class="pln"> stmt</span><span class="pun">=</span><span class="pln">conn</span><span class="pun">.</span><span class="pln">createStatement</span><span class="pun">();</span><span class="pln">

  </span><span class="com">//创建表结构,插入一些测试数据</span><span class="pln">
  stmt</span><span class="pun">.</span><span class="pln">execute</span><span class="pun">(</span><span class="str">"CREATE TABLE EmployeesData( "</span><span class="pln">
     </span><span class="pun">+</span><span class="pln"> </span><span class="str">"Id INT NOT NULL GENERATED ALWAYS AS IDENTITY, "</span><span class="pln">
     </span><span class="pun">+</span><span class="pln"> </span><span class="str">"Name VARCHAR(255), "</span><span class="pln">
     </span><span class="pun">+</span><span class="pln"> </span><span class="str">"Salary INT NOT NULL, "</span><span class="pln">
     </span><span class="pun">+</span><span class="pln"> </span><span class="str">"Location VARCHAR(255), "</span><span class="pln">
     </span><span class="pun">+</span><span class="pln"> </span><span class="str">"PRIMARY KEY (Id))"</span><span class="pun">);</span><span class="pln">
  stmt</span><span class="pun">.</span><span class="pln">execute</span><span class="pun">(</span><span class="str">"INSERT INTO EmployeesData(Name, Salary, Location) "</span><span class="pln">
     </span><span class="pun">+</span><span class="pln"> </span><span class="str">"VALUES (Amit, 30000, Hyderabad), "</span><span class="pln">
     </span><span class="pun">+</span><span class="pln"> </span><span class="str">"(Rahul, 39000, Lucknow), "</span><span class="pln">
     </span><span class="pun">+</span><span class="pln"> </span><span class="str">"(Renuka, 50000, Hyderabad), "</span><span class="pln">
     </span><span class="pun">+</span><span class="pln"> </span><span class="str">"(Learnfk, 15000, Vishakhapatnam), "</span><span class="pln">
     </span><span class="pun">+</span><span class="pln"> </span><span class="str">"(Kalyan, 40000, Hyderabad), "</span><span class="pln">
     </span><span class="pun">+</span><span class="pln"> </span><span class="str">"(Trupthi, 45000, Vishakhapatnam), "</span><span class="pln">
     </span><span class="pun">+</span><span class="pln"> </span><span class="str">"(Raghav, 12000, Lucknow), "</span><span class="pln">
     </span><span class="pun">+</span><span class="pln"> </span><span class="str">"(Suchatra, 33000, Vishakhapatnam), "</span><span class="pln">
     </span><span class="pun">+</span><span class="pln"> </span><span class="str">"(Rizwan, 20000, Lucknow)"</span><span class="pun">);</span><span class="pln">

  </span><span class="com">//执行SQL语句</span><span class="pln">
  </span><span class="typ">String</span><span class="pln"> query</span><span class="pun">=</span><span class="str">"SELECT Location, SUM(Salary) from EmployeesData GROUP BY Location"</span><span class="pun">;</span><span class="pln">
  </span><span class="typ">ResultSet</span><span class="pln"> rs</span><span class="pun">=</span><span class="pln">stmt</span><span class="pun">.</span><span class="pln">executeQuery</span><span class="pun">(</span><span class="pln">query</span><span class="pun">);</span><span class="pln">
  </span><span class="kwd">while</span><span class="pun">(</span><span class="pln">rs</span><span class="pun">.</span><span class="kwd">next</span><span class="pun">())</span><span class="pln"> </span><span class="pun">{</span><span class="pln">
     </span><span class="typ">System</span><span class="pun">.</span><span class="kwd">out</span><span class="pun">.</span><span class="pln">println</span><span class="pun">(</span><span class="str">"Location: "</span><span class="pun">+</span><span class="pln">rs</span><span class="pun">.</span><span class="pln">getString</span><span class="pun">(</span><span class="lit">1</span><span class="pun">));</span><span class="pln">
     </span><span class="typ">System</span><span class="pun">.</span><span class="kwd">out</span><span class="pun">.</span><span class="pln">println</span><span class="pun">(</span><span class="str">"Sum of salary: "</span><span class="pun">+</span><span class="pln">rs</span><span class="pun">.</span><span class="pln">getString</span><span class="pun">(</span><span class="lit">2</span><span class="pun">));</span><span class="pln">
     </span><span class="typ">System</span><span class="pun">.</span><span class="kwd">out</span><span class="pun">.</span><span class="pln">println</span><span class="pun">(</span><span class="str">" "</span><span class="pun">);</span><span class="pln">
  </span><span class="pun">}</span><span class="pln">



}
}

在执行上述程序时，您将获得以下输出-

Location: Hyderabad
Sum of salary: 120000

Location: Lucknow
Sum of salary: 71000


Location: Vishakhapatnam
Sum of salary: 93000

参考链接

www.learnfk.com/derby/apach…

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